1. Field of the Invention
The invention is generally related to abrasive airblast systems and is specifically directed to air drying and treatment means for such systems.
2. Discussion of the Prior Art
Abrasive blasting is a well known operation for cleaning or preparing a surface by forcibly propelling a stream of abrasive material against it. Abrasive blasting systems have been around for over a century, with an early blasting processed patented by Benjamin Chew Tilghman on Oct. 18, 1870. In such operations the abrasive material is made up of small granular or particulate matter which is introduced into a pressurized air stream for dry blasting or a fluid for wet blasting.
Dry abrasive blasting applications are typically powered from an air compressor and are known as airblast systems. Most applications involve a pressurized vessel that contains the abrasive and meters it into the compressed air stream, primarily by gravity flow.
Typical pressure blast or airblast systems provide for the feed of abrasive media to be controlled independently of blast pressure. The abrasive media is generally not recycled in open air systems and must be continually supplied during an operation. In this type of system the abrasive media is stored in a hopper or pot and is fed into the nozzle air stream from the hopper in a separate air stream. The separate air stream primarily is used to fill the void caused in the hopper as, the abrasive material gravity flows out of the hopper and into the blast stream.
The blasting equipment usually consists of but is not limited to a hand-held nozzle that directs a stream of the granular abrasive media particles toward a work surface or a work piece. The media flows from the hopper into a mixing chamber in order to transport the media to the nozzle where it is subject to a high velocity air stream that propels it toward the work.
Moisture in the compressed, pressurized air causes the granular abrasive inside the airblast system to “clump” and not flow or not flow consistently. An airblast system requires abrasives to flow consistently for optimum performance. If the “clumping” is excessive the granular abrasive will not gravity flow from the hopper into the mixing chamber and will result in an intermittent flow, causing an inconsistent surface finish. An airblast system requires abrasives to gravity flow consistently for optimum performance.
Currently, compressed air is treated with a compressed air cooler or a compressed air dryer. The air cooler typically consists of an air cooled radiator or heat exchanger to utilize ambient air to cool the hotter compressed air. Then the air is further treated with an apparatus to remove the resulting condensation from cooling the compressed air. In the prior art there are two preferred methods for moisture removal. In the first system, the condensate droplets are first removed from the air stream via centrifugal force. The air is introduced tangentially into a knockout tank to induce centrifugal motion. Second, the compressed air flows through a coalescing pad that will trap moisture droplets. At this point the compressed air has been cooled and virtually all of the moisture is removed.
In the second system a dryer is used to remove the moisture. The most prevalent dryer is a deliquescent dryer. In a typical system the deliquescent dryer has the same elements as the air cooler with the addition of a deliquescent bed. After the air is cooled and moisture removed, the compressed air flows through a deliquescent bed which will remove water vapor to achieve a relative humidity of 55%. This equates to approximately 20° F. dew point suppression. For example, if the compressed air is cooled to 115° F., the deliquescent bed will remove enough water vapor to achieve a relative humidity of 55% or a dew point of 95° F. This means that the compressed air has to further cool below 95° F. degrees for condensation to form.
In prior art systems, the blast units are either connected to air coolers or to dryers. During blasting, 99% of the air consumption is for the blast nozzle(s) and 1% goes to the pressure vessel or abrasive hopper. This 1% is basically to fill the volume of the abrasive that has drained out of the vessel or hopper for blasting. However, the 1% of air that goes into the hopper is required to be much dryer than the 99% of air that flows through the nozzle. This is because once the abrasive mixture enters the nozzle flow line the propulsion force and speed of particle flow is such that “clumping” is not an issue.
The prior art systems typically equally treat and dry all of the air flowing through the system even though only 1% is required to be at an elevated dry condition.